Piezoelectric materials have been well known for many decades, one most common example being a piezoelectric ceramic, lead zirconate titanate.
Electroactive polymers (EAP) are an emerging class of materials within the field of electrically responsive materials. EAPs can work as sensors or actuators and can easily be manufactured into various shapes allowing easy integration into a large variety of systems.
Materials have been developed with characteristics such as actuation stress and strain which have improved significantly over the last ten years. Technology risks have been reduced to acceptable levels for product development so that EAPs are commercially and technically becoming of increasing interest. Advantages of EAPs include low power, small form factor, flexibility, noiseless operation, accuracy, the possibility of high resolution, fast response times, and cyclic actuation.
The improved performance and particular advantages of EAP material give rise to applicability to new applications.
An EAP device can be used in any application in which a small amount of movement of a component or feature is desired, based on electric actuation. Similarly, the technology can be used for sensing small movements.
The use of EAPs enables functions which were not possible before, or offers a big advantage over common sensor/actuator solutions, due to the combination of a relatively large deformation and force in a small volume or thin form factor, compared to common actuators. EAPs also give noiseless operation, accurate electronic control, fast response, and a large range of possible actuation frequencies, such as 0-20 kHz.
Devices using electroactive polymers can be subdivided into field-driven and ionic-driven materials.
Examples of field-driven EAPs are dielectric elastomers, electrostrictive polymers (such as PVDF based relaxor polymers or polyurethanes) and liquid crystal elastomers (LCE).
Examples of ionic-driven EAPs are conjugated polymers, carbon nanotube (CNT) polymer composites and Ionic Polymer Metal Composites (IPMC).
Field-driven EAP's are actuated by an electric field through direct electromechanical coupling, while the actuation mechanism for ionic EAP's involves the diffusion of ions. Both classes have multiple family members, each having their own advantages and disadvantages.
FIGS. 1 and 2 show two possible operating modes for an EAP device.
The device comprises an electroactive polymer layer 14 sandwiched between electrodes 10, 12 on opposite sides of the electroactive polymer layer 14.
FIG. 1 shows a device which is not clamped. A voltage is used to cause the electroactive polymer layer to expand in all directions as shown.
FIG. 2 shows a device which is designed so that the expansion arises only in one direction. The device is supported by a carrier layer 16. A voltage is used to cause the electroactive polymer layer to curve or bow.
The nature of this movement for example arises from the interaction between the active layer which expands when actuated, and the passive carrier layer. To obtain the asymmetric curving around an axis as shown, molecular orientation (film stretching) may for example be applied, forcing the movement in one direction.
The expansion in one direction may result from the asymmetry in the electroactive polymer, or it may result from asymmetry in the properties of the carrier layer, or a combination of both.
In a sensing application, deformation induced by an external force to be sensed can give rise to a measurable change in impedance. Alternatively, some electroactive materials such as piezoelectric materials, generate electrical charge in response to the external force stimulus.
In certain applications, an array of sensors can be useful for example to measure the contact area between a device and the human body. For instance, the contact area between a touch panel and a human finger is measured in the system described in US 2012/0086651. This document describes a touch panel which includes a passive matrix of piezoelectric polymer sensors. The shape of the sensing electrodes is a plurality of electrode lines arranged on the upper and lower surfaces of the piezoelectric layer, in such a manner that the upper and lower electrode lines are perpendicular to each other.
In order to determine unique position patterns in such a matrix it is necessary to read out sensor elements individually, or to sequentially scan rows and columns. This requires complicated wirings (which individually connect all pixels) and/or electronics (row and column switches).
This complexity can be avoided by performing a parallel read-out of rows and columns, but the penalty is that it is no longer possible to determine unique position patterns. Thus, a conventional passive matrix concept does not enable unique position patterns to be determined with parallel read-out of rows and columns.
There is therefore a need for a simple matrix addressing scheme which enables a detection of a pattern applied to a sensor array.